1. Field of the Invention
This invention relates to a method for detecting a prospective abnormal pattern from an image.
2. Description of the Prior Art
Techniques for reading out a radiation image of an object and reproducing a visible radiation image have heretofore been carried out in various fields. With the techniques, a radiation image of an object, which has been recorded on a recording medium, such as a stimulable phosphor sheet or X-ray film, is read out, an image signal is thereby obtained, and the obtained image signal is subjected to appropriate image processing and then used for reproducing a visible image on a display device, or the like. In particular, recently, various digital radiography techniques, which utilize computers and are referred to as computed radiography (CR), have been proposed and applied to clinical diagnoses, or the like.
In cases where a stimulable phosphor sheet is employed as the recording medium, the radiation image of an object is stored as a level of energy during the exposure of the stimulable phosphor sheet to radiation. In such cases, the image signal representing the radiation image is obtained by exposing the stimulable phosphor sheet to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and detecting the emitted light with a photoelectric read-out means.
In the CR, a read-out sensitivity and contrast adjusting function, which is referred to as the exposure data recognizer (EDR), is employed. With the EDR, a preliminary read-out operation (hereinbelow referred to as the "preliminary readout") is carried out in order to approximately ascertain the image information of a radiation image, which has been stored on a stimulable phosphor sheet. An electric image signal, which is to be used in reproducing a visible image capable of being used for diagnostic purposes, or the like, is obtained from a final read-out operation (hereinbelow referred to as the "final readout"). During the preliminary readout, stimulating rays are used, which have an energy level lower than the energy level of the stimulating rays used in the final readout. In accordance with the image information having been obtained from the preliminary readout, read-out conditions for the final readout, such as a read-out sensitivity (hereinbelow often referred to as the S value) and a latitude (hereinbelow often referred to as the L value), are adjusted such that the visible image reproduced from the image signal, which is obtained from the final readout, can have good image quality, e.g. an appropriate density and an appropriate contrast, and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
A method for utilizing the preliminary readout and the final readout has been disclosed in, for example, U.S. Pat. No. 4,527,060. With the disclosed method, the final readout is carried out under the read-out conditions, which have been adjusted with the EDR. The image signal obtained from the final readout is fed into an image processing means. In the image processing means, the image signal is processed in accordance with the portion of the object the image of which is recorded, the conditions under which the image is recorded, or the like, such that a visible image suitable for the diagnostic purposes, or the like, can be reproduced. A visible image is then reproduced from the processed image signal on a photographic material, or the like.
Also, a processing method, wherein the preliminary readout need not be carried out, time loss due to the preliminary readout is thereby eliminated, and the processing and the apparatus are thereby kept simple, has been proposed in, for example, U.S. Pat. No. 5,046,147. With the proposed processing method, the detection range for the light emitted by a stimulable phosphor sheet is set to be sufficiently wide (for example, approximately 4 orders of ten), and the entire radiation image is thereby read out to obtain an image signal. From the obtained image signal, an appropriate read-out sensitivity and an appropriate latitude are determined. Thereafter, in accordance with the determined read-out sensitivity and the determined latitude, the obtained image signal is transformed into an image signal, which is equivalent to the image signal, which would be obtained if the image were again read out under the conditions of the determined read-out sensitivity and the determined latitude.
With the proposed processing method, the setting values of the photoelectric read-out means with respect to the amount of the stimulating rays irradiated to the stimulable phosphor sheet, the sensitivity, the dynamic range, and the like, need not be set again, and an image signal necessary to reproduce an appropriate image can be obtained.
Further, particularly for medical diagnoses of human bodies, techniques referred to as the computer aided diagnosis of medical images (CADM) have been proposed, which aim at more positively utilizing the features of the digital radiography.
The techniques for the computer aided diagnosis of medical images, or the like, assist in making diagnoses by reading patterns in an image at the sites of medical treatment. Specifically, in the past, medical specialists visually read patterns in radiation images having been reproduced on recording media, such as X-ray film, display devices, such as cathode ray tube (CRT) display devices, or the like, and made efforts in order to find out abnormal tumor patterns, which represented cancers, or the like, small calcified patterns, which are smaller than the tumor patterns and have a density lower than the density of them, and the like, in the early stages of the diseases. (The tumor patterns, the small calcified patterns, and the like, will hereinbelow be referred to as the abnormal patterns.) However, in such cases, there is the risk that the abnormal patterns are left unnoticed or are misunderstood due to subjective judgments, depending on differences between the image understanding capabilities of persons, who view the radiation images.
Therefore, the techniques for the computer aided diagnosis of medical images aim at preventing the persons, who view the radiation images, from failing to notice the abnormal patterns and misunderstanding the abnormal patterns, and thereby aim at enabling the persons to make the efficient and accurate diagnosis of an illness. For such purposes, with the techniques for computer aided diagnosis of medical images, a prospective abnormal pattern, which is considered as being an abnormal pattern, is detected. Also, a marking is put on the detected portion in order to arouse an attention of the person, who views the radiation image. Alternatively, characteristics of the detected prospective abnormal pattern are indicated quantitatively as materials, which are useful for objective judgments of the person, who views the radiation image. [Reference should be made to "Detection of Tumor Patterns in DR Images (Iris Filter)," Collected Papers of The Institute of Electronics and Communication Engineers of Japan, D-II, Vol. J75-D-II, No. 3, pp. 663-670, March 1992; and "Extraction of Small Calcified Patterns with A Morphology Filter Using A Multiply Structure Element," Collected Papers of The Institute of Electronics and Communication Engineers of Japan, D-II, Vol. J75-D-II, No. 7, pp. 1170-1176, July 1992.]
With the iris filter processing, the gradients of image signal values, which are represented by the density values of a radiation image, are calculated as gradient vectors, and information representing the degree of centralization of the gradient vectors is utilized in order to detect a prospective tumor pattern. Specifically, it has been known that, for example, in a radiation image recorded on a negative X-ray film, the density values of a tumor pattern are slightly smaller than the density values of the surrounding image areas. The gradient vector at an arbitrary picture element located in the tumor pattern is directed toward the vicinity of the center point of the tumor pattern. On the other hand, in an elongated pattern, such as a blood vessel pattern, gradient vectors do not centralize upon a specific point. Therefore, the distributions of the directions of the gradient vectors in local areas may be evaluated, and a region, in which the gradient vectors centralize upon a specific point, may be detected. The thus detected region may be taken as a prospective tumor pattern, which is considered as being a tumor pattern. The processing with the iris filter is based on such fundamental concept. Steps of algorithms of the iris filter will be described hereinbelow.